Fully differential operational amplifiers (op amps) require stable differential and common-mode operation. As such, modern op amps are designed with separate differential and common-mode feedback compensation networks to stabilize both differential and common-mode operation of the op amp. The compensation in each compensation network is provided by capacitors, where the capacitors either perform differential or common-mode compensation.
FIG. 1 shows conventional operational amplifier 100. Op amp 100 operates between +VCC and −VCC rails, where differential signals fed to differential inputs Vip and Vin are amplified and output at differential outputs Vop and Von during differential operation. Amplification of the differential signals is provided by flowing current from the +VCC rail, through the individual cascoded transistor pairs (e.g., pmos transistor 110 and nmos transistor 115, and pmos transistor 120 and nmos transistor 125) and current sink 130, and into the −Vcc rail. Differential feedback is provided by differential feedback loop 140, which feeds back voltages from the differential outputs to the differential inputs via differential feedback circuit 145.
Common-mode feedback loop 150 is used to set the operating point of op amp 100 during common-mode operation. Resistors 151 and 152 provide a voltage divider for generating an average voltage (e.g., an average of the differential outputs) at common-mode output Vocm, where the average voltage is fed back to the common-mode feedback circuit 155 for comparison with a reference voltage Vref also fed to circuit 155. In response to the comparison, circuit 155 will generate a common-mode control signal fed to common-mode input Vicm. The voltage at Vicm is used to adjust the bias of pmos transistors 110 and 120, thereby shifting the voltage at the differential outputs to set the operating point of op amp 100.
As shown in FIG. 1, differential feedback loop 140 and separate common-mode feedback loop 150 utilize separate compensation capacitors to stabilize both modes of operation. For example, differential feedback loop 140 uses compensation capacitors 146 and 147 to stabilize differential operation, while common-mode feedback loop 150 uses compensation capacitor 158 to stabilize common-mode operation. Given the large area required on integrated circuit dies to implement capacitors, the compensation networks of op amp 100 require large die area, thereby limiting use in integrated circuits with smaller allotted die areas. Additionally, the separate differential and common-mode compensation networks of op amp 100 increase the design and design verification time as both networks must be individually optimized. Further, given that both compensation networks couple to virtual ground as shown in FIG. 1, the noise of op amp 100 is increased, thereby limiting use in designs requiring low noise.